1. Field of the Invention
This invention relates generally to a system and method for the compensation of image motion during aircraft reconnaissance missions. In particular, this invention utilizes electronic methods for minimizing the loss of image sharpness caused by image motion during the exposure intervals of cameras using electro-optical area array image sensors.
2. Related Art
Aerial reconnaissance systems have undergone a dramatic transition in the past two decades with the replacement of photographic film by electro-optic image sensors. With the advent of wafer-scale focal planes that provide sufficient coverage and resolution, reconnaissance systems are being designed to utilize electro-optic sensors configured as large format area arrays. These electro-optic ("EO") reconnaissance imaging systems most often employ charge-coupled devices ("CCDs") operating in the visible and near-infrared regions of the electromagnetic spectrum to capture the image of the target or scene. The ability to operate in a real-time environment and in low ambient light conditions are just a few of the reasons why electro-optical-based reconnaissance imaging systems are increasingly replacing film-based reconnaissance systems.
One of the more frequently encountered problems in designing aerial reconnaissance imaging systems is determining the most effective method of compensating for image smear or blurring. Typically, smearing occurs when low ambient light conditions prevent an imaging system from using sufficiently short exposure times, resulting in a blurred image due to the forward motion of the aircraft. In other words, smearing occurs as a result of the relative motion between a scene or target to be imaged and the imaging system. Therefore, in order to prevent the degradation of the information contained in a recorded image, an ideal reconnaissance imaging system must utilize some means of image motion compensation ("IMC") for image smear.
For example, if the target being imaged is directly below the aircraft, the relative motion of the image will be directly proportional to the forward velocity of the aircraft and inversely proportional to the aircraft's altitude and thus easily compensated. However, any practical IMC method must also factor in the problems associated with operating the reconnaissance imaging system at a non-perpendicular and oblique angle from the direction of motion of the aircraft. In these oblique angle instances, typically referred to as side oblique or forward oblique orientations, the images of objects in the field of view of the imaging system that are a closer distance to the aircraft (i.e., near field objects) move relatively faster than the images of objects at a farther distance from the aircraft (i.e., far field objects).
The manner in which the scene image appears to move, and the extent of how much it moves, depends upon the flight parameters (e.g., altitude, velocity, etc.), the imaging system parameters (e.g., exposure time, lens focal length, etc.), and upon the orientation of the target area with respect to the imaging system (e.g., vertical, side oblique or forward oblique). The presence of these image motions at the focal plane introduces smear in the image. Depending on flight and system parameters, if the motions remain uncompensated a greater or lesser degradation in image quality and resolution results. In order to reduce such smearing of the image, various techniques for forward motion compensation (FMC) or IMC have been developed.
Some conventional methods of image compensation employed in reconnaissance imaging systems are mechanically based. For example, one early mechanical technique involved translating film in the same direction and velocity as the image motion, so that the image appears stationary on the moving film. Additionally, this same technique can be employed by moving a CCD (as opposed to film) proportional to the velocity and direction of the image motion. U.S. Pat. No. 4,908,705, issued Mar. 13, 1990 to Wight et al., which is incorporated herein by reference, discloses such a technique where the array physically moves fore and aft in the same direction as the apparent motion of the image to reduce the smear. Another related mechanical technique involves translating the focussing lens relative to the motion of the image. Again, the image at the focal plane appears to be stationary using this technique. However, this technology does not account for differences in relative image motion between near-field and far-field objects.
With the increased use of CCD arrays instead of film in many reconnaissance imaging systems, IMC methods are now based either solely on electronic means or on a combination of mechanical and electronic methods. One such electronic method is column-segmented forward motion compensation.
Column-segmented FMC is typically designed to work with either a between-the-lens shutter or a focal plane shutter camera. Here the area of the imager is broken up into some number of segments, with each segment being a group of columns. The size of the segments is dictated by the magnitude of the differential motion in the column direction for segments across the array and the practicality of adding ever more segments.
An example of column-segmented FMC is disclosed in U.S. Pat. No. 5,155,597 issued Oct. 13, 1992 to Lareau et al., which is also incorporated herein in its entirety by reference. Briefly, Lareau discloses an apparatus and method for the correction for the image motion in side oblique operation by transferring the charge in the CCD array along column groups at different transfer rates, corresponding to the depression angle of the column segment of the imager with respect to the horizon reference of the vertical field-of-view.
As described in the above Lareau reference, column-segmented FMC is useful for side oblique operation when the column direction is oriented in the same direction as the aircraft line of flight. In this case image motion is greatest along column segments for receiving images from scene objects near the aircraft.
With forward oblique camera orientation, the column direction is typically co-planar with the flight direction, resulting in large variations of image motion from top to bottom of each column segment. In this case, IMC clock frequencies for all column segments are typically set at an average value, which cannot correct for the range of motion along the columns. Thus, the column segmentation method described above offers little or no advantage for forward oblique applications.
A major factor to consider in implementing an EO reconnaissance system is minimizing complexity in device design and processing, which is essential to obtaining sufficient yield to make the device economically viable. The column segmented FMC method described above exemplifies a device requiring complex design features and device processing (e.g., accessing clock electrodes in each column segment). What is needed is a device design with minimum design and processing complexity that allows IMC in one or more of the commonly used camera orientations. In addition, it is also desirable for a device design which, if required to be more complex, can provide IMC for side oblique, forward oblique, or any camera orientation requiring combined side and forward oblique compensation.